Combine biscuit mix, cocoa, sugar, eggs, milk, butter or margarine and vanilla in large bowl.
Beat on low speed with electric mixer 1/2 minute to blend ingredients.
Increase the speed to medium and beat for 4 minutes.
Scrape the bowl often with a rubber spatula.
Turn batter into a buttered 9x9x2 inch baking pan.
Bake in moderate oven (350°) 30 minutes, or until center of cake springs back when lightly pressed with finger.
Cool cake in pan on wire rack.
Combine instant coffee powder with 2 tea spoons warm water in a small bowl.
Stir in vanilla frosting.
Swirl frosting on top of cake.
Garnish with shaved semi sweet chocolate, if you wish.
Cut cake in squares.
Remaining cake may be covered and stored in the pan.